1. Field of the Invention
The present invention relates to a laser diode suitably applicable to applications demanding light output having a stable polarization direction and a method of manufacturing the same.
2. Description of the Related Art
Types of laser diodes include an edge-emitting laser diode, a surface-emitting diode and the like. The surface-emitting laser diode emits light in a direction orthogonal to a substrate, and a large number of devices are allowed to be arranged in a two-dimensional array on a single substrate. Therefore, the surface-emitting laser diode has received attention as a light source for digital copying machine or printer.
In the surface-emitting laser diode, for example, a pair of multilayer reflecting mirrors are formed on a semiconductor substrate, and an active layer including a light emission region is included between the pair of multilayer reflecting mirrors. A current narrowing layer is arranged on one of the multilayer reflecting mirrors so as to increase current injection efficiency into the active layer and to reduce a threshold current. In the surface-emitting laser diode, a current is confined by the current narrowing layer, and then the current is injected into the active layer, thereby light is emitted in the active layer, and while the light is repeatedly reflected by the pair of multilayer reflecting mirrors, the light is emitted from the light emission opening of one electrode (a p-side electrode) as laser light.
Such a surface-emitting laser diode typically has such nonuniformity that a polarization direction varies due to device variations, or such instability that the polarization direction is changed by output or environmental temperature. Therefore, an issue arises in the case where the surface-emitting laser diode is applied to a polarization-dependent optical device such as a mirror or a beam splitter. For example, in the case where the surface-emitting laser diode is used as a light source for digital copying machine or printer, variations in the polarization direction cause a difference in imaging position or output, thereby blur or color unevenness occurs.
Therefore, to solve such an issue, some techniques of stabilizing a polarization direction in one direction by arranging a polarization controllability function in the surface-emitting laser diode have been reported.
For example, a technique using a special inclined substrate which has a (311) plane as a normal and is made of gallium-arsenic (GaAs) is known. In the case where a surface-emitting laser diode is formed using such a special inclined substrate, gain characteristics with respect to the [−233] direction are enhanced, and the polarization direction of laser light is controllable to this direction. In addition, the polarization ratio of laser light is very high, so this technique is effective to stabilize the polarization direction of the surface-emitting laser diode in one direction.
Moreover, in Japanese Patent No. 2891133, a technique of controlling polarization by reducing the size of a section of a post structure to be smaller than the mode size of light is disclosed.
Moreover, in Published Japanese Translation. No. 2001-525995 of PCT international application, a technique of forming a discontinuity in a part of a metallic contact layer which does not have an influence on characteristics of laser light emitted from a light emission opening so as to obtain polarization in a direction parallel to a boundary of the discontinuity is disclosed.
Further, in Japanese Unexamined Patent Application Publication No. 2008-016824, a technique of arranging a pair of oxidation regions so as to face each other with a light emission region in between, thereby obtaining polarization in a direction orthogonal to a direction where the pair of oxidation regions face each other is disclosed.